Generally, during a period in which signal outputted from an output terminal of a shift register unit is at a low level, the signal outputted from the output terminal of the shift register unit is subject to be disturbed by an input clock signal and thus noise is generated in the signal. In order to suppress noise, the shift register unit generally comprises a pulling-down thin film transistor (TFT) for pulling down the signal outputted from the output terminal. A pulling-down control node is connected to a gate of the pulling-down TFT, and is configured to control to turn on the pulling-down TFT, so as to pull down a level of a gate driving signal at the output terminal.
Known shift register units at least have the following problems: generally, most of the time, the pulling-down control node connected to the gate of the pulling-down TFT is kept to be at a high level and thus the pulling-down TFT is kept to be turned on, resulting in an occurrence of a relative large drift in a threshold voltage of the pulling-down TFT. The pulling-down TFT could not be turned on if the threshold voltage of the pulling-down TFT is increased continuously, such that a function of noise suppression cannot be achieved and performance of a whole shift register is deteriorated.